The invention relates generally to chemical sensors and more particularly to carbon monoxide sensors.
The reaction of Carbon Monoxide (CO) with a variety of molybdenum compounds is reported as early as 1910 [C. Zenghelis, Z. Anal. Chem, 40, 429, (1910)] and the literature is reviewed in 1935 by J. Schmidt, "Das Kohlenoxyd", Akad. Verlag, Leipzig, p 186, (1935). It is further presented in "Spot Tests in Inorganic Analysis" by F. Feigel in 1937. [F. Feigel, V. Anger, R. Oesper, "Spot Tests in Inorganic Analysis", Elsevier Publishing Company, New York, p. 168 (1972)].
In this relevant literature, three (3) basic equations are pointed out: EQU Mo.sup.+6 +CO.fwdarw.Mo.sup.+3 +CO.sub.2 (1) EQU Pd.sup.+2 +CO+H.sub.2 O.fwdarw.Pd.sup.0 +CO.sub.2 +2H.sup.+ (2) EQU Pd.sup.0 +Mo.sup.+6 .fwdarw.Pd.sup.+2 +Mo.sup.+3 (3)
The reaction in Equation (1) is very slow and, therefore, a catalyst (Pd.sup.+2) is used. The Pd.sup.+2, even in very small quantities, adsorbs and simultaneously gets reduced by CO as shown in Equation (2). Pd.sup.0, in turn, enters into the reaction, Equation (3), producing molybdenum blue. Thus, a slightly yellow solution is changed to a blue color with the intensity of the blue color being directly related to CO exposure. As presented in Equations (1), (2) and (3), the reaction is not reversible and, therefore, has limited application in sensor technology.
To make the system reversible, therefore, there must be a secondary
The invention relates generally to chemical sensors and more reaction which converts Mo.sup.+3 back to Mo.sup.+6, i.e., an oxidizer must be present.
The question of the reversible CO sensor is addressed by M. K. Goldstein in U.S. Pat. No. 5,063,164. That patent suggests several possible chemical recipes for this type of sensor, but does not address the criteria or requirements for a successful reversible sensor; nor does it address the chemistry or mechanisms to make the CO sensor completely specific.
Goldstein shows a solid state CO sensor having five components: (1) palladium salt, (2) molybdenum and/or tungsten salt or acid salt, (3) copper salt, (4) cyclodextrin molecular encapsulant which encapsulates at least one but not all of the other components, and (5) chloride salt, all impregnated into a substrate.